1. Field of the Invention
This invention relates to novel dielectric ceramics, and more specifically, to dielectric ceramics which permits free control of the temperature coefficient of the resonant frequency to a positive or a negative value, and is suitable for use in constructing high frequency oscillators and filters.
2. Description of the Prior Art:
High frequency dielectric ceramics used as resonators or the like are generally required to have a high specific dielectric constant, a high unloaded Q and a low absolute value of the temperature coefficient of the resonant frequency. In recent years, complex oxides having a perovskite-type crystal structure having the composition represented by A(B'.sub.1/3 B".sub.2/3)O.sub.3 wherein A and B' represent a divalent cation, and B" represents a pentavalent cation have attracted attention as high frequency dielectric ceramics, and for example, Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 and Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 are known.
High frequency dielectric ceramics are also used as resonators in oscillators for high frequencies such as microwaves and millimeter waves. When such an oscillator is used, for example, in satellite broadcasting, it is desired to increase its temperature stability by minimizing the absolute value of the temperature coefficient of the oscillating frequency, i.e. to a value within .+-.1.8 ppm/.degree. C. (the oscillating frequency 10.678 GHz.+-.1.5 MHz, -30 to +50.degree. C.). The temperature coefficient of the oscillating frequency is determined not only by the temperature coefficient (.tau..sub.f) of the resonant frequency of the dielectric ceramics, but also by various factors such as the temperature coefficient of FET, the coefficient of thermal expansion of the substrate or the coefficient of thermal expansion of the metallic casing. No technique, however, has been established by which the oscillator is designed with all these factors taken into consideration and the temperature coefficient of the oscillating frequency is accurately controlled. The best method presently practiced is to select dielectric ceramics having a suitable temperature coefficient of the resonant frequency and incorporate it in an oscillator whereby the temperature coefficients, etc. of other component parts are compensated and consequently, the temperature coefficient of the oscillating frequency of the oscillator is controlled. It is desired therefore not only to minimize the absolute value of the temperature coefficient of the resonant frequency of high frequency dielectric ceramics, but also to control the temperature coefficient freely to a desired positive or negative value in the production of the dielectric ceramics.
The aforesaid conventional high frequency dielectric ceramics, however, have the disadvantage that particularly, in the microwave and millimeter wave regions, the temperature coefficient of the resonant frequency can assume only a nearly constant value for a given material of which the dielectric ceramics is made, and its value cannot be controlled freely to a desired positive or negative value in the production of the ceramics.